The present invention relates generally to starting and controlling the speed of a two spool gas turbine engine and, in particular, to providing a motor/generator to add or subtract power to the gas generator spool of a two spool gas turbine engine.
Two spool gas turbine engines are often desirable solutions as prime movers for various transportation applications. These engines consist of a gas generator spool and a power turbine spool. Most of the output power of the engine is taken by mechanically connecting a load to the power turbine spool. Very often, the load has a high inertia and engine response to load changes is relatively slow.
For example, when the load connected to the power turbine spool consists of an electrical generator, the electrical loads connected to the generator may be suddenly applied or removed. Because the gas turbine engine cannot respond as quickly as desired, a rapid load transient may cause operational and reliability problems, such as compressor surge and high turbine temperature. When a load is suddenly removed, the speed of the power turbine spool may increase above the desirable operating maximum speed, resulting in an overspeed condition. Conventional two spool gas turbine engines are often designed, at the expense of weight, cost and reliability, to accommodate the engine speed response to sudden load changes and to avoid compressor surge, overtemperature and overspeed.
In another example the load connected to the power turbine spool consists of the fan (or propeller) of an aircraft turbofan (or turboprop) gas turbine engine. Rapid load changes can cause operational and reliability problems as described above.
Mose et al., U.S. Pat. No. 4,771,606, disclose a helper drive apparatus for a turbine drive system in which a load is mainly driven by a turbine and a helper motor is used for assisting the drive for the load, particularly when the cost of electric power is favorable compared to the cost of steam power which drives the turbine. The Mose invention, however, requires the availability of external (utility) power and does not address the problem of controlling the shaft speed of a two spool gas turbine engine, especially during generator load transients.
Kure-Jensen et al., U.S. Pat. No. 5,301,499, disclose a system and method for anticipating and controlling overspeed in a combined cycle gas and steam turbine engine. The Kure-Jensen patent discloses maintaining the engine speed within desirable limits through a control system which anticipates overspeed conditions and jointly controls the fuel flow valve of a gas turbine and the steam control valve of a steam turbine. The Kure-Jensen invention, however, cannot be applied to a simple gas turbine engine.
Birch et al., U.S. Pat. No. 5,635,768, disclose a system for maintaining the engine speed within desirable limits by controlling the external electric load while incorporating a motor to control the fuel supplied to the engine. The Birch invention however cannot be applied to applications where the gas turbine engine is the only primary source of electrical power and limiting the electrical load is not practical.
As can be seen, there is a need for an improved method and apparatus for starting and controlling the speed of a two spool gas turbine engine such that compressor surge, overtemperature and overspeed may be minimized.